1. Field of the Invention
The present invention relates to a screw compressor which includes a pair of male and female screw rotors meshing with each other, and permits easy change of a size of a discharge opening even when operation conditions such as a compression ratio change.
2. Description of the Related Art
A performance of a screw compressor largely depends on a form (area and shape) of a discharge opening structured by an axial port (a discharge opening in the axial direction) and a radial port (a discharge opening in the radial direction). However, the size of the discharge opening of the screw compressor is usually fixed according to the operation conditions. In other words, this size of the discharge opening is determined by a design capacity ratio Vi defined by the following equation (1).Vi=V1/V2  (1)where:
V1: maximum suction capacity of rotor grooves, and
V2: final discharge capacity of rotor grooves.
A relationship between this design capacity ratio Vi and a design pressure ratio {dot over (m)} determined by operation conditions is represented by the following equation (2):{dot over (m)}=P2/P1=(V1/V2)n  (2)where:
P1: suction pressure,
P2: final pressure in rotor grooves, and
n: adiabatic exponent.
Thus, if the optimal size of the discharge opening is determined for a discharge pressure Pd such that a relationship: Pd=P2 holds, it is possible to attain maximum efficiency. Though it is desirable to determine the form of the discharge opening according to the operation conditions of the screw compressor as described above, a reduction in number of man-hours required for producing the casing has conventionally been regarded as important, and the size of the discharge opening is not changed even if the operation conditions change in some degree.
However, as a need for enhancement in performance and efficiency of screw compressors has recently been increasing, simply supplying screw compressors including a fixed discharge opening cannot meet this need, therefore, a screw compressor permitting easy change of the form of a discharge opening according to operation conditions while the number of man-hours required for producing is reduced has been sought for.
For an application of a compressor used for drawing gas from an oilfield, for example, as a result of the drawing of the reserved gas for a medium or long period, the quantity of the reserved gas decreases, and the gas pressure to be drawn therefore decreases, resulting in a large displacement in the operation conditions from initial design conditions. Since an optimal form of the discharge opening cannot be secured due to the displacements from the initial design conditions to the operation conditions, a loss in power is thus generated, and a problem of a deteriorated efficiency can occur. Moreover, a deficiency in power of a driving source may require a reproduction of an entire compressor unit. In order to cope with the problem as described above, a need is increasing for a compressor which permits easy change of the form of a discharge opening.
Under this technical background, as a conventional example, Japanese Laid-Open Patent Publication (Kokai) No. H7-208362 proposes a screw compressor in which a radial discharge port 19a is formed by thin casting, and a thin portion can be removed for operation at a low pressure ratio and at a high rotation speed as shown in FIG. 6. As a result, since the size of the discharge port 19a can be easily optimized, the same cast casing can allegedly provide a high efficiency under a wide range of operation conditions.
However, for the discharge port of the screw compressor according to the conventional example, though the number of man-hours required for production is reduced compared with that required for technologies before this conventional example, a manufacturing process for removing a thin portion (thickness: h2) of the discharge port formed by casting according to the operation conditions still needs a large load in terms of machining/production, this conventional example cannot sufficiently meet the need for a compressor permitting easy change of the size of the discharge port according to the operation conditions, while reducing the number of man-hours required for the production.